Endplay
For many years, tapered roller bearing adjustment consisted of mounting a hub, two tapered roller bearings, two washers, and an oil seal onto the axle spindle to be retained with a spindle nut. The assembly was adjusted by rotating the spindle nut to achieve the condition known as desired “endplay.” This term describes the amount of lateral movement of the hub assembly with respect to the axle spindle resulting from a measure of looseness attributable to the established position of the spindle nut. This movement has been measured and expressed by the number of thousandths on an inch (0.001″). Typically the acceptable range of endplay has been on the order of 0.001″ to 0.005″.
Preload
The term “preload” was adopted by the roller bearing industry to identify the pounds of force applied against the bearings after the spindle nut had been tightened to the extent endplay was no longer discernible. Unfortunately, it was not possible to measure the amount of preload force that occurs within the wheel-end assembly unless some form of internal sensor was placed into appropriate contact with either of the tapered roller bearings. Although this has been done on an experimental basis, it is not feasible for vehicles in highway operations due to excessive cost.